The present invention relates to an electron microscope device, by which it is possible to observe and examine a scanning electron image and an optical image of a specimen. In particular, the invention relates to an electron microscope device, by which an optical image can be observed at the same time during electron scanning operation.
A scanning electron microscope (SEM) is designed in such manner that an electron beam is projected for scanning over a specimen, and the electron issued from the specimen by the projection of the electron beam is detected and a scanning electron image is acquired as to properties of a surface of the specimen.
On the other hand, a field angle of the electron beam is very small, and it is very difficult to project the electron beam to a specific position unless a projecting position is identified and recognized in advance.
For this reason, an electron microscope device is provided with an optical microscope, which has lower magnification as compared with the electron microscope. First, an illumination light (white light) is projected to the specimen, and the specimen is observed by the optical microscope, and a projected position is specified. Then, a change over from the optical microscope to the electron microscope is performed, the electron beam is projected to scan over the projected position of the specimen and the specimen is observed.
In case it is wanted to acquire a scanning electron image, electrons emitted from the specimen are launched into a fluorescent substance. A light emitted from the fluorescent substance is converted to an electric signal by a photoelectric conversion element, and a scanning electron image is acquired based on this electric signal. In case an optical image is acquired by using the optical microscope, a light reflected from the specimen is received by a photoelectric element for the optical microscope, and is converted to the electric signal, and the optical image is acquired based on this electric signal.
However, in case where electrons obtained by scanning of the electron beam are compared with a light reflected from the specimen, a energy level of the reflected light is extremely higher than an energy level of electrons. When a reflected light from the specimen enters the photoelectric element, which detects the electron beam, the photoelectric element is saturated or S/N ratio is extremely low, and the electron beam cannot be detected.
Therefore, it has been practiced in the past that the microscopes are changed completely between in an observation by the optical microscope and in an observation by the electron microscope so that the electron microscope is not used for observation during the observation by the optical microscope, and that the optical microscope is not used for observation during the observation by the electron microscope.
For instance, the optical microscope and the electron microscope are separated from each other, and an optical axis of the optical microscope and an optical axis of the electron microscope are set in a known specific relation. When a table on which the specimen placed is moved between the optical microscope and the electron microscope, a position to observe an optical image is associated with a position to observe a scanning electron image.
In this respect, in a conventional type electron microscope, there has been a problem that the structure is very complicated and the scanning electron image and the optical image cannot be observed at the same time.
An electron microscope, by which the scanning electron image and the optical image can be observed and examined at the same time, is disclosed in JP-A-4-280053.
In the electron microscope described in JP-A-4-280053, an optical system of the electron microscope and an optical system of the optical microscope have the same optical axis. An illumination light and an electron beam are projected at the same time. In the signals from the electron detector, signals of the illumination light are removed as DC components, and signals containing only electrons are extracted.
However, as described above, the DC components of the illumination light are extremely in higher amount compared with signal components, and it is very difficult to extract signals containing only electrons.